The present invention relates to a power window device for automatically lowering and raising a window member by operating a switch.
In the prior art, a power window device is installed in a door of a vehicle to facilitate the lowering and raising of a window glass (opening and closing of a window) in the door. For each door of a vehicle, the power window device includes a window switch, which is operated by a vehicle occupant when lowering or raising the window glass of the door, and a motor, such as a DC motor, for lowering or raising the window glass. When a window switch is operated, the associated motor is driven to produce rotation that lowers or raises the corresponding window glass.
In such a power window device, if a large load is applied to the motor when lowering or raising the window glass, the motor may become locked. For example, the motor becomes locked when the window glass reaches a fully open position or fully closed position. In such a case, the motor stops moving the window glass. Japanese Laid-Open Patent Publication No. 8-254071 describes a power window device including a shunt resistor arranged between the motor and ground. A temperature detector detects the temperature of the shunt resistor. If a large load is continuously applied to the window glass when the motor is driven to lower or raise the window glass such as when the window glass has already reached the fully closed position, the current flowing through the shunt resistor increases. This heats and increases the temperature of the shunt resistor. When the temperature detector detects an excessive temperature increase in the shunt resistor, the motor is inactivated.
Another type of power window device includes a positive coefficient heater (PTC) thermistor, which is arranged in the vicinity of the motor, to cope with large loads applied to the motor. If the motor is continuously driven after the window glass reaches the fully open or closed position, the temperature of the motor increases. When the motor temperature becomes excessively high, the resistance of the PTC thermistor suddenly increases and stops the flow of current to the motor. This PTC thermistor effect inactivates the motor and stops the lowering or raising of the window glass.
Referring to FIG. 4, continuous or repetitive operation of the window switch after the window glass reaches the fully open or fully closed position may result in the occurrence of the PTC thermistor effect. In such a case, the window glass cannot be moved when the vehicle occupant operates the window switch. Furthermore, much time may be necessary for the motor to cool down until the PTC thermistor returns to a normal state so as to enable the vehicle occupant to operate the window glass again. As a result, the vehicle occupant may erroneously determine that there is an anomaly in the power window device even though the power window device is functioning normally. Accordingly, when the vehicle occupant operates the window switch, it is desirable that the heating of the motor be suppressed to avoid the PTC thermistor effect. The same problem occurs when using the above shunt resistor and temperature detector to monitor excessive increase in the motor temperature.